The Darrow-Yannet diagram is used to analyze body fluid compartment changes. A patient with primary hyperaldosteronism (Conn's syndrome) develops marked sodium and water retention. How do ECF and ICF change?

• A ECF expands isosmotically, ICF remains unchanged due to co-retention of water with sodium ✓
• B ECF expands, ICF contracts (water moves out of cells due to ECF hypertonicity)
• C ICF expands, ECF contracts due to aldosterone-driven intracellular Na+ accumulation
• D Both compartments contract due to urinary potassium losses exceeding sodium gains

Explanation

Aldosterone causes renal Na+ retention, but because osmoreceptors trigger proportional ADH release and water retention, plasma osmolality remains near-normal (isosmotic expansion). Therefore, both Na+ and water are retained isosmotically into the ECF, expanding it without creating an osmotic gradient to draw water from cells — ICF volume is unchanged. This is the 'escape phenomenon.' Contrast with hypertonic saline administration, where ECF hypertonicity would draw water from cells and shrink ICF.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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